In recent years, along with a tendency to high brightness and whitening of a light-emitting diode (LED) element, a light-emitting device employing a LED element has been used for backlights of mobile phones, liquid crystal TVs, liquid crystal displays, etc.
However, in such a light-emitting device, there has been a problem such that along with a tendency to high brightness of a LED element, heat generation is also increasing, and the temperature increases excessively, whereby adequate emission brightness has not necessarily been obtained. Further, in order to obtain a high light extraction efficiency, a technology has been proposed to dispose a silver reflection film on e.g. a substrate surface to mount a LED element, but in such a light-emitting device, there has been a problem of so-called light intensity deterioration such that the light extraction efficiency deteriorates as compared with the initial stage since a silver reflection film is deteriorated by oxidation or sulfurization due to heat or permeated gas. Accordingly, as a light-emitting device employing a light-emitting element such as a light-emitting diode element, one having high heat dissipation property and light reflection efficiency, and further having less deterioration in light intensity in long-term use has been desired.
In order to solve these problems, e.g. Patent Document 1 discloses a technology relating to a light-emitting device having a structure which can suppress occurrence of delamination between a molded member forming a sealing layer and a substrate made of a ceramic material with a high heat dissipation property, such as alumina, aluminum nitride or mulite, and further having a construction in which a high-melting point metal film having a high adhesion with a substrate and a metal film having a high reflectivity are used in combination as a reflection film to improve adhesion to a substrate and reflectivity of a metal reflection film. Further, Patent document 2 discloses a technology of employing a sealing resin having a low gas permeability, as a sealing resin to seal a semiconductor light-emitting element, an electrical conductor electrically connected therewith or a reflection film constructed on a substrate. Further, in a light-emitting device employing a low temperature co-fired ceramic substrate (hereinafter referred to as LTCC) having a high heat dissipation property, a silver reflection film disposed on a substrate is covered with an overcoat glass film, for the purpose of preventing deterioration of light intensity.
Here, a LED element having a relatively low output, which has been commonly used, has a structure in which light is emitted not only from a light extraction side but also from a surface (rear surface of a LED element) mounted on a substrate of a light-emitting device, toward the substrate. Therefore, in order to reflect this light to the light extraction side, the mounting surface of the substrate is made of a high-reflectance material, and further a silicone-type die bond material having a high light transmittance is employed as a die bond material to bond the LED element with the substrate.
For example, Patent Document 3 discloses a technology relating to a LED light-emitting device, in which a substrate material having a high thermal conductivity and less change in reflection efficiency, specifically one having an aluminum plate subjected to alumite treatment or one having a copper plate subjected to aluminizing treatment, is used for a substrate, and to such a substrate, a LED chip is die-bonded by a transparent paste (silicone die bond material). Further, in such a light-emitting device using a conventional LED element, in consideration of the heat dissipation property, an electroconductive die bond material made of a thermosetting resin containing e.g. silver particles is preferred as a die bond material, but the electroconductive die bond material is not usually used since the light extraction efficiency tends to deteriorate due to low reflectance of the electroconductive die bond material.
On the other hand, a recently used LED element having a high output exceeding 0.5 W and a large heat generation, has a structure in which a metal layer is formed on a rear surface (surface to be mounted on a substrate) as an opposite side of a light extraction side having a light emitting layer formed thereon. Since a material (such as aluminum) having not only a high thermal conductivity but also a high reflectance is used for the metal layer, it also functions as a reflection film, and light emitted from a light emitting layer of an upper layer hits on the metal layer (reflection film) on a rear surface and reflected upward. Accordingly, it is not necessary to dispose a material having a high reflectance, on a substrate just under the mounting portion of the LED element. Further, light never passes through the rear surface due to the reflection film, and therefore it is possible to use an electroconductive bonding material for bonding to a substrate, in consideration of the heat dissipation property.
However, although such a LED element having a metal reflection film on a rear surface opposite to a light extraction side is bonded on a substrate by using a electroconductive die-bond material, except for this aspect, the LED element has been used for a light-emitting device having the same construction as the conventional light-emitting device, such that the LED element is mounted e.g. on the above conventional substrate for mounting light-emitting element having the same construction as a LED element having no metal reflection film, specifically, on a substrate of ceramics having a high thermal conductivity such as alumina or aluminum nitride, or on an overcoat glass film covering a silver reflection film formed on a LTCC substrate. Accordingly, the light extraction efficiency is insufficient, and it is impossible to obtain light having high brightness corresponding to high output of the light-emitting element.
Accordingly, in a light-emitting device mounting a light-emitting element such as a LED element having a metal reflection film on a surface (a rear surface of the LED element) as an opposite side of a light extraction side, it has been desired to develop a light-emitting device having a structure which has a high heat dissipation property and high light reflection efficiency thereby to obtain an excellent light extraction efficiency and suppresses deterioration of light extraction efficiency due to deterioration with time.